In multiple exposure high dynamic range (ME-HDR) imaging, an image sensor captures multiple image frames with different integration times, where the integration times have fixed ratios between them. The frames are then combined together to produce a high dynamic range (HDR) image. The integration times in most image sensors are controlled by an electronic rolling shutter (ERS). Capturing images with an ERS-based image sensor may produce a spatial flicker when a scene is illuminated by alternating current (AC) sources. The spatial flicker typically appears as intensity variations or ‘bands’ across the image (i.e. spatial artifacts).
The number of bands may be affected by the AC source frequency and the exposure time. The amplitude of the intensity variation is dependent on the waveform driving the scene illumination. The flicker can also vary across different color channels depending on the type of illumination. In ME-HDR, all of these factors result in frames exhibiting spatial flicker with phase differences and amplitude variations between multiple frames. When the frames are combined together, color artifacts may be observed in the HDR image in addition to the intensity bands.
A conventional method for avoiding flicker is to adjust the integration time to be an integer multiple of half the flicker period. This technique can eliminate flicker for the longest-exposure image (T1). Adjusting the integration time for the shorter-exposure images (T2, T3 . . . TN) while simultaneously maintaining the necessary integration ratios between them, however, does not produce flicker-free images, since the shorter integration times are almost always less than one flicker period.